The invention relates to a method for controlling a hydraulic system of a working machine.
The invention is applicable on working machines within the fields of industrial construction machines, in particular wheel loaders and articulated haulers. Although the invention will be described with respect to a wheel loader, the invention is not restricted to this particular machine, but may also be used in other working machines, such as dump trucks, excavators or other construction equipment.
A working machine is provided with a bucket, container or other type of implement for lifting, carrying and/or transporting a load.
For example, a wheel loader has a load arm unit for raising and lowering the implement, such as a bucket. The load arm unit comprises a number of hydraulic cylinders for movement of the load arm and the implement attached to the load arm. A pair of hydraulic cylinders is arranged for lifting the load arm and a further hydraulic cylinder is arranged on the load arm for tilting the implement.
The wheel loader which usually is frame-steered has also a pair of hydraulic cylinders for turning/steering the wheel loader by pivoting a front part and a rear part of the wheel loader relative to each other.
In addition to the hydraulic cylinders, the hydraulic system of the wheel loader preferably comprise a first pump (working cylinder pump) for providing hydraulic fluid to the hydraulic cylinders of the load arm unit and a second pump (steering cylinder pump) for providing hydraulic fluid to the steering cylinders. In order to optimize the hydraulic system and use the pumps in an effective way the steering cylinder pump can also be used for providing hydraulic fluid to the hydraulic cylinders (working cylinders) of the load arm unit.
Such a hydraulic system usually has a priority valve to ensure that the steering cylinders are supplied with hydraulic fluid by means of the steering pump, and only if there is an excess of hydraulic fluid the steering pump can also supply hydraulic fluid to the working cylinders.
A disadvantage with prior art hydraulic systems using such a priority valve is instability in the steering function, in particular when supplying hydraulic fluid solely to the steering cylinders (and not the working cylinders). It has been shown that the tendency to instability in prior art hydraulic system of the current type is most significant when using the steering cylinder pump for providing hydraulic fluid solely to the steering cylinders, whereas when the steering cylinder pump is used for providing hydraulic fluid also to the working cylinders the hydraulic system works smoother since less adjustment of the displacement of the steering cylinder pump is required.
In such a hydraulic system the steering cylinder pump is controlled to pressurize the hydraulic fluid to a pressure above the load pressure, and the priority valve in turn lowers the pressure to a pressure less than the pump pressure. The flow from the steering cylinder pump, via the priority valve, is directed to a control valve which in turn provides the steering cylinder with hydraulic fluid, and also provides the steering cylinder pump as well as the priority valve with a LS-pressure corresponding to the load pressure of the steering cylinder.
Undesired phase shifts of the pump and the priority valve pressures may arise due to the fact that the displacement of the pump is adjusted. If a very small flow or no flow is required, and thus the displacement of the pump is controlled accordingly, then it takes a certain time to adjust the displacement of the pump so as to increase the flow again when an increased flow is required. This may cause phase shifts of the steering cylinder pump pressure and the priority valve pressure and imply instability in the hydraulic system, i.e. the pump pressure level and the pressure level after the priority valve may start to “oscillate” up and down without finding a stable state.
It is desirable to provide a method defined by way of introduction, which method counteracts instability in the hydraulic system at the same time as the pumps of the hydraulic system can be used efficiently.
By the provision of a method according to an aspect of the present invention having a first control mode allowing a flow of hydraulic fluid from the second hydraulic machine to the second actuator, wherein the pump pressure of the second hydraulic machine is selected based on the load pressure of the second actuator independently of the load pressure of the first actuator, and having a second control mode, which is used provided that the requested flow to the first actuator exceeds a predetermined threshold value, allowing a flow of hydraulic fluid from the second hydraulic machine to the first actuator, wherein the pump pressure of the second hydraulic machine is selected based on the highest value of the load pressure of the second actuator and the load pressure of the first actuator, for example a steering function can be controlled in the first control mode without being negatively affected by a work function which in turn implies increased stability, and the second hydraulic machine can be used in the second control mode to provide additional hydraulic fluid to the work function.
In addition, by selecting the pump pressure of the second hydraulic machine based on the load pressure of the second actuator independently of the load pressure of the first actuator in the first control mode, different pump pressures can be used for the first and second actuators, thereby eliminating energy losses which otherwise would arise when the first and second actuators require different pump pressures.
The wording “predetermined threshold value” comprises automatically and manually selected values. The threshold value can be a variable value in order to achieve various conditions for using the second control mode. The threshold value can be a function of one or more parameters. For example, the threshold value can be selected differently for different operation modes. Preferably, the threshold value is selected on the basis of the flow capacity of the first hydraulic machine.
According to one embodiment of the invention, the hydraulic fluid from the second hydraulic machine is provided to the second actuator and to the first actuator via a priority valve in order to give priority to the second actuator (which can be a steering function) over the first actuator (which can be a work function such as lift or tilt for an implement), and in the first control mode the priority valve is controlled to be substantially fully opened to the second actuator. Hereby, any losses of energy related to a pressure drop over the priority valve can be decreased or eliminated and any instability related to pressure regulation by the priority valve can be counteracted or avoided in the first control mode.
The invention also relates to a computer program and a computer readable medium for performing the steps of the method according to the invention.
Further advantages and advantageous features of the invention are disclosed in the following description and in the dependent claims.